The invention relates generally to cryogenic fluid dispensing systems and, more particularly, to a system for heating cryogenic liquid during transfer to a dispensing system bulk storage tank based upon the pressure within the bulk storage tank.
Current alternative fuels include cryogenic substances such as Liquified Natural Gas (LNG). Cryogenic substances have a boiling point generally below xe2x88x92150xc2x0 C. A use device, such as an LNG-powered vehicle, may need to store LNG in an on-board fuel tank with a pressure head that is adequate for the vehicle engine demands. That is, the LNG can be stored in a saturated state on board the vehicle in order to maintain the desired pressure while the vehicle is in motion. This saturation generally occurs by heating the LNG prior to its introduction into the vehicle tank.
LNG is typically dispensed from a bulk storage tank to a vehicle tank by a pressurized transfer. This may be accomplished through the use of a pump, pressurized transfer vessels or a straight pressure transfer from the bulk storage tank at a higher pressure to a vehicle tank at a lower pressure.
A common method of saturating cryogenic liquids, such as LNG, is to saturate the LNG as it is stored in a conditioning tank of a dispensing station. In some instances, the conditioning tank may also be the bulk storage tank of the dispensing station. The LNG may be heated to the desired saturation temperature and pressure by removing LNG from the conditioning tank, warming it, and reintroducing it back into the conditioning tank. The LNG may be warmed, for example, by heat exchangers as illustrated in U.S. Pat. Nos. 5,121,609 and 5,231,838, both to Cieslukowski, and 5,682,750 to Preston et al. Alternatively, the LNG may be heated to the desired saturation temperature and pressure through the introduction of warmed cryogenic gas into the conditioning tank. Such an approach is illustrated in U.S. Pat. Nos. 5,421,160, 5,421,162 and 5,537,824, all to Gustafson et al.
A disadvantage of such re-circulation or warmed gas arrangements, however, is that when a single interim or the bulk tank is used as the conditioning tank, continuous dispensing of saturated LNG is not possible. More specifically, saturated LNG is not available for dispensing during refill of the conditioning tank and while the newly added LNG in the conditioning tank is being conditioned. As a result, saturated LNG may not be available for dispensing for significant periods of time.
While the above difficulties may be overcome by providing a pair of interim dispensing station transfer or conditioning tanks, such a system has to be tailored in dimensions and capacities to specific site conditions, that is, the amount of fills, pressures expected, etc. As a result, deviations from the design conditions still results in problems for such a system.
Saturating LNG in a conditioning tank by re-circulation or warmed gas is also not optimal in terms of time efficiency. Transferring the LNG from a transport, such as a tanker truck, to the bulk storage tank takes an extended pumping time. After the transfer is complete, and the bulk storage tank has been refilled, conditioning of the transferred LNG must occur which takes an additional period of time. Time efficiency would be improved if the conditioning occurred during the transfer of LNG to the bulk tank.
Another approach for saturating the LNG prior to delivery to the vehicle tank is to warm the liquid as it is transferred to the vehicle tank. Such an approach is known in the art as xe2x80x9cSaturation on the Flyxe2x80x9d and is illustrated in U.S. Pat. No. 5,787,940 to Bonn et al. wherein heating elements are provided to heat the LNG as it is dispensed. U.S. Pat. Nos. 5,687,776 to Forgash et al. and 5,771,946 to Kooy et al. also illustrate dispensing systems that use heat exchangers to warm cryogenic liquid fuel as it is transferred to a vehicle. While such prior art xe2x80x9cSaturation on the Flyxe2x80x9d systems remove the difficulties associated with saturating the dispensing station vessel, they can be complex and, as a result, more costly. The additional components may also have additional maintenance needs.
U.S. Pat. No. 5,373,702 to Kalet et al. presents an LNG delivery system, indicated in general at 50 in FIG. 1, whereby a vehicle fuel tank is initially filled with unheated LNG from a storage tank 52 via lines 54 and 58, pump 56 and coupling 60 to purposely collapse the vapor head therein. The vehicle fuel tank features a spray head positioned in its vapor space through which the LNG from the delivery system flows. The liquid dispensing line 58 includes a pressure sensor 72 which provides an indication to a microprocessor 70 when the liquid level in the vehicle tank reaches the spray head. The microprocessor then manipulates valves 66 and 68 so that LNG is routed through line 62 and a heat exchanger 64. As a result, natural gas vapor is produced and delivered to the vehicle fuel tank so that the LNG therein is saturated. The vehicle includes an overflow tank which receives LNG that is displaced from the vehicle fuel tank as the natural gas vapor is added and saturation occurs. A disadvantage of such an arrangement, however, is the requirement that the vehicle include an overflow tank. This adds to the vehicle cost, weight and complexity.
Accordingly, it is an object of the present invention to provide a system enables continuous dispensing of cryogen.
It is another object of the present invention to provide a system that conditions cryogen without the need for recirculating the cryogen from a conditioning tank.
It is another object of the present invention to provide a system that conditions cryogen without the need for dual conditioning tanks.
It is still another object of the present invention to provide a system that heats cryogenic liquid as it is transferred to a storage tank.
It is still another object of the present invention to provide a system that heats cryogenic liquid as it is being transferred based upon the pressure in the destination tank.
These and other objects and advantages will be apparent from the following specification.
A system constructed in accordance with the present invention dispenses cryogenic liquid to a use device tank from a bulk storage tank containing a supply of cryogenic liquid. A dispensing line is in communication with the bulk storage tank and is adapted to communicate with the use device tank. A pump and heater are in circuit with the dispensing line. A system control device, such as a microprocessor, is in communication with the pump and heater so that cryogenic liquid may be dispensed, and selectively heated as it is dispensed, to the use device tank.
A liquid level sensor and a pressure or temperature sensor communicate with the use device tank and the system control device so that the liquid level and temperature or pressure of cryogenic liquid initially in the use device tank may be determined. The system control device uses this information to calculate the amount of heat and cryogenic liquid that must be added to the use device tank to optimally fill the use device tank. The system control device then operates the heater and pump to fill the use device tank with cryogenic liquid saturated as required. Unheated cryogenic liquid is preferably initially added to the use device tank so that the vapor head therein is collapsed. Heat may then be added to the cryogenic liquid stream as it is dispensed prior to the completion of the fill to saturate the liquid and rebuild pressure in the use device tank.
The system may alternatively include only a liquid level sensor in communication with the use device tank. The liquid initially in the use device tank is assumed to be saturated and at the pressure required by the use device when such an embodiment is selected.
The pump is preferably a positive displacement pump and is preferably submerged in cryogenic liquid housed in a sump. The heater may include a heat exchanger, electric heater, cryogenic gas or other heating arrangement.
An embodiment of the system of the present invention includes a replenishing tank or a transport tank containing a supply of cryogenic liquid, a bulk storage tank and a pump in circuit there between so that a stream of cryogenic liquid flows from the replenishing tank to the bulk storage tank when the pump is operated. The replenishing tank may be the tank of a delivery vehicle or a fixed transfer tank. A heat exchanger and a parallel by-pass line are in circuit between the pump and the bulk storage tank. A flow control valve is positioned within the by-pass line and is adjustable to direct a portion of the cryogenic liquid stream to the heat exchanger so that it is warmed and rejoined with a remaining portion of the stream passing through the by-pass line. The warmed stream then flows to the bulk storage tank.
A pneumatic controller senses pressures within the bulk storage tank via a pressure line that is in communication with the head space of the bulk storage tank. The pneumatic controller automatically adjusts the flow control valve based upon the pressures detected within the bulk storage tank so that the bulk storage tank is ultimately filled with saturated cryogenic liquid at the desired pressure.
An alternative embodiment of the system of the present invention omits the pressure line and the pneumatic controller and instead includes a pressure sensor in communication with the head space of said bulk storage tank and an actuator that automatically adjusts the flow control valve. A programmable controller is in communication with the pressure sensor and the actuator so that the latter automatically adjusts the flow control valve based upon the pressures detected within the bulk storage tank. As a result, the bulk storage tank is ultimately filled with saturated cryogenic liquid at the desired pressure. The programmable controller may also be in communication with the pump so as to adjust its operating speed based upon pressures detected within the bulk storage tank.
The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, provide a more complete understanding of the nature and scope of the invention.